The present invention is in the field of pollution and contaminant removal.
This invention relates to apparatus useful in the removal of air pollutants. More specifically, this invention relates to apparatus useful in mitigating major air pollutants (i.e., SOx and NOx) and trace toxins from coal-fired combustors.
The residual solids resulting from various flue gas desulfurization (FGD) processes, including scrubber sludge, contain significant portions of unreacted sorbent. Unless these solids are treated, they will be sent to landfills, thus increasing the cost associated with sorbent requirements and waste disposal.
Nitrogen oxides (NOx) are emitted when fossil fuels such as coal, natural gas, or oil are burned in air. NOx emissions have attracted increased attention in recent years as more is learned about their role in acid rain, smog, visibility impairment and global climate change. About half of all nationwide NOx pollutants come from automobiles, whereas coal-burning utility boilers contribute about 25% of the total. The 1990 Clean Air Act amendments require all coal-fired utility boilers over a certain size to reduce NOx by about 50%. In addition, it is expected that regulations affecting the emission of NOx will get tougher in the future and power plants will need to reduce emissions even further. Another serious problem may occur in integration, when trace metals and NOx may contaminate the catalyst.
In coal-fired power plants, disposal of coal combustion products such as ashes and wet/dry FGD products is a serious concern. Most of these solid wastes are sent to landfills for disposal. Studies have shown that, when treated properly, these solid waste products can be used beneficially in a number of applications.
It is therefore an object of the invention to provide a cost-effective method and apparatus for reducing the residual solids produced during the mitigation of major air pollutants and trace toxins from coal-fired combustors by recycling the unreacted sorbent contained in those solids.
Although described with respect to the field of mitigating major air and trace toxins from coal-fired combustors, it will be appreciated that similar advantages may obtain in other applications of the present invention. Such advantages may become apparent to one of ordinary skill in the art, in light of the present disclosure or through practice of the invention.
The present invention includes a reactivation technique developed from a fundamental understanding of the pore structural properties of both CaCO3 and Ca(OH)2, and the evolution of pore structure with calcination and sintering.
Integration of a SOx removal process of the present invention with SCR technology for NOx removal offers an attractive alternative to post-combustion flue gas clean-up technologies as it not only reduces the emission of acidic pollutants but also reduces the amount of solid-waste generated. The combined SOx/NOx technology of the present invention integrates a novel ash reactivation process for SOx removal with proven SCR technology for NOx removal. The coal fired power plants that use high sulfur coals can be encouraged to continue using these coals by retrofitting to include the inventive process for advanced, cost effective NOx and S02 removal combined with reduced solid waste generation and increased dry FGD product utilization.
A recycling of spent sorbent and fly ash mixture into the spray dryer may result in substantial improvements in reagent utilization and SO2 removal. Substantial reactions may occur between the fresh Ca(OH)2 and recycled fly ash from spray dryer, resulting in the formation of hydrated calcium silicates. Their subsequent reaction with SO2 may lead to increased efficiency.
Although not limited to the theory of the invention, the key to the high reactivity of a fresh or partially utilized sorbent may lie in its open initial internal structure and subsequent pore structure evolution under high temperature conditions. The present reactivation technique may be used for spent and under-utilized sorbents, and benefits from the pore structural properties of both CaCO3 and Ca(OH)2, and the evolution of pore structure with calcination and sintering. The present invention includes a suspension-based carbonation process in which the unreacted CaO is converted into calcium carbonate (CaCO3) instead of calcium hydroxide (Ca(OH)2).
Along with reactivation of unreacted CaO, this process provides a better distribution/exposure of available calcium than the reactivated spent sorbent from hydration alone. The process of the present invention has been successfully applied to the reactivation of two partially utilized sorbents generated in the laboratory, and has been further demonstrated to reactivate two commercial ash samples under bench-scale conditions.
Accordingly, the present invention includes devices and systems useful in removing air pollutants. This invention also includes machines or instruments using these aspects of the invention. The present invention may be used to upgrade or retrofit existing machines or instruments using methods and components known in the art.
The present invention also includes methods and processes using the devices of the present invention. The methods and processes of the present invention may be applied using procedures and protocols known and used in the arts to which they pertain.
In broadest terms, the present invention includes a method of removing SOx and trace metals from a gaseous waste stream from coal combustion, where the coal combustion generates an untreated gaseous waste stream containing SOx and trace metals, and the treatment of the gaseous waste stream generates a source of limestone, lime or slaked lime, comprising the steps: (a) admixing carbon dioxide with the source of limestone, lime or slaked lime and water so as to carbonate the limestone, lime or slaked lime, whereby a carbonated sorbent is produced; and (b) contacting the gaseous waste stream containing SOx and trace metals with the carbonated sorbent, so as to remove SOx and trace metals from the gaseous waste stream. The water may additionally contain at least one substance selected from the group consisting of surfactants and modifiers. The gaseous waste stream containing SOx and trace metals may be contacted with the carbonated sorbent in a circulating fluidized bed reactor. The gaseous waste stream may additionally contain NOx species, and may be contacted with a catalyst adapted to remove the NOx species following step (b). The flow of clean flue gas containing carbon dioxide may be obtained from contacting the gaseous waste stream with a catalyst adapted to remove NOx species following step (b).
The present invention also includes, in broadest terms, a system for removing SOx and trace metals from a gaseous waste stream from coal combustion, whereby the coal combustion generates an untreated gaseous waste stream containing SOx and trace metals, and the treatment of the gaseous waste stream generates a source of limestone, lime or slaked lime and a flow of clean flue gas containing carbon dioxide, comprising: (a) a coal-burning facility producing a source of an untreated gaseous waste stream containing SOx and trace metals, and an apparatus for removing SOx so as to generate a source of limestone, lime or slaked lime; (b) a carbonation reaction container for admixing the flow of said clean flue gas containing carbon dioxide with the source of limestone, lime or slaked lime with and water so as to carbonate said limestone, lime or slaked lime, thereby producing a carbonated sorbent; (c) a dryer adapted to remove water from the carbonated sorbent; and (d) a sorbent reaction container for contacting the gaseous waste stream containing SOx and trace metals with the carbonated sorbent, so as to remove SOx and trace metals from the gaseous waste stream.
The apparatus for removing SOx may be selected from the group consisting of wet, dry and wet-dry scrubbers. The system may additionally comprise a catalytic reaction container for contacting the gaseous waste stream with a catalyst adapted to remove NOx species following treatment in the sorbent reaction container. The catalytic reaction container may comprise a selective catalytic reduction catalyst. The system may additionally comprise a conduit adapted to conduct a flow of clean flue gas from the sorbent reaction container to the carbonation reaction container. The system may also comprise a conduit adapted to conduct a flow of clean flue gas from the catalytic reaction container to the carbonation reaction container.
The system may additionally comprise a conduit adapted to conduct the source of limestone, lime or slaked lime from the coal-burning facility to the carbonation reaction container. The system may also comprise a conduit adapted to conduct the untreated gaseous waste stream from the coal-burning facility to the sorbent reaction container. The system may contain a conduit adapted to conduct a flow of water to the carbonation reaction container. The system may include at least one particle separator, and may include a heat exchanger adapted to supply heat from the flow of clean flue gas to the dryer. The sorbent reaction container may also comprise a circulating fluidized bed reactor.
The present invention also includes, in broadest terms, a method of preparing a sorbent from limestone, lime or slaked lime generated from the removal of SOx from a gaseous waste stream from coal combustion, comprising the steps: obtaining said limestone, lime or slaked lime; and (b) admixing carbon dioxide and water with the limestone, lime or slaked lime so as to carbonate the limestone, lime or slaked lime, thereby producing a carbonated sorbent.